Such control devices are used, in particular, for controlling the position of a flap or the like in a heater system or air conditioning system for the cabin of a motor vehicle. Devices controlled by means of reduced pressure are presently preferred to devices controlled by means of cable because the operate more smoothly. In addition, in the event of a breakdown in which the source of reduced pressure in no longer operative, the flap or the like is automatically returned to a failsafe or safety position. For example, this could correspond to a position in which the windscreen is demisted or defrosted. In contrast, if a control cable is broken, it is not possible to ensure that the flap or the like returns to a failsafe position.
Known reduced pressure control devices for moving a movable member between three positions comprise a housing whose interior is divided into sealed chambers by means of two flexible and deformable membranes which are fixed to the housing at their peripheries. Each of the membranes has a rigid cup mounted thereon, one of which is mounted to move the movable member outside the housing in some suitable manner, and the other of which is connected to the first by a telescopic or retractable type of link member serving to establish predetermined maximum and minimum distances between the cups. Each sealed chamber has a respective orifice or pipe fitting capable of being selectively connected to atmospheric pressure or to a source of reduced pressure. A first return spring is interposed between the membranes, and a second return spring is interposed between the second membrane and an end wall of the housing. The movable member can thus be moved to a first position by leaving both orifices connected to atmospheric pressure, to a second position by connecting one of the orifices to the reduced pressure, and to a third position by connecting both orifices to the reduced pressure.
It has been found that the telescopic or retractable link member complicates assembly of the device, thus increasing its cost, and also adds to the length of the device.
Preferred embodiments of the invention avoid these drawbacks and do so in a simple, effective and cheap manner.